The invention relates to a method and arrangement for automatically focussing an objective onto a subject or test object. More particularly, the invention relates to automatic focussing expedients involving autocollimation and the use of gratings for generating an electrical signal which can be used to control the operation of means for automatically adjusting the setting of the objective.
It has been many times proposed (for example in Federal Republic of Germany published patent application DT-OS 1,908,687) to use gratings or rasters located in the autocollimation beam path of a camera objective or autocollimation objective. Specifically, these known expedients have involved the projection of the image of one grating onto the subject or test object. Then the image, on the subject, of the first grating is projected, using autocollimation, onto the second grating. The extent to which the image of the first grating registers with the second grating is detected photoelectrically, by detecting the intensity of the light projected onto the second grating, and an electrical signal is generated for controlling electromechanical means for effecting proper or optimal focussing of the objective.
The known proposals for effecting automatic focussing in this way have not, to our knowledge, met with any practical success. The reason for this, at least in part, is that the use of two discrete gratings or rasters is relatively complicated and expensive, because they must be properly adjusted relative to each other and kept properly adjusted with very great precision.